Assuming your JSON is well formed and all fields are extracted, try putting your field name within single or double quotes
index="xxx" source="E:\\path\\REST\\test*" metricId!=9011737 metricId!=9012116 | stats sum('metricValues{}.sum') as total by metricId
*OR*
index="xxx" source="E:\\path\\REST\\test*" metricId!=9011737 metricId!=9012116 | rename 'metricValues{}.sum' AS s | stats sum(s) as total by metricId
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